Your room dimensions are quite similar to mine. And I have the larger 3.6's on the short wall. With planars, you don't have to be as concerned about side wall reflections. Several feet is OK, but more is obviously better.
As far as power, yes they aren't as efficient as your DT's, but with a robust receiver amps with true power in the 120+ watt @ 8 ohm and capable of 4 ohms, even if the mfg doesn't say it specifically, the smaller Maggies are definitely doable. I used the old MG-1's with 60 watts for many years, and 1.6's with the older Pioneer flagships 49 & 59txi receivers. Going to 3.6's, an ext amp is definitely recommended.
If you have a budget receiver, like $250-500, then Maggies are not the way to go. If you have a good upper end receiver, say Denon 4311, maybe even 3311, Pioneer SC55/57 or similar from Onkyo, the 1.7's will be doable. The amps have to be good enough to drive 4 ohms. I know from personal experience Pioneer's SC models can do the job. And I agree with the other poster, more power is still better with Maggies.
And get a sub, even the 1.7's, rated at 40 hz, won't get you into the 20 hz range where explosions, etc will have the most impact.
You might want to seriously consider a short wall setup. Plenty of room to bring the speakers out and plenty of room for a 7.1 rear speaker setup. The side walls will be in the null zone with the speakers slightly toed in, so even if you have them 2 ft off the side, it won't significantly degrade the imaging with a bunch of reflected sound. I wouldn't say that with a std speakers, but planars have more of a figure 8 sound pattern, so not as critical on the sides. And if you want, you can always hang an acoustic wall panel on the sides at 1st reflection points.
In their price range, 1.7's are a true bargain in audio. So I understand why you're interested. Make sure you have decent amplification, a subwoofer, and a bit of effort on placement, you can make it work in your room. Long wall will also work very well and may be ideal, as long as you bring them off the front wall several ft. Like DonH50 says, you could get by at 1 ft but don't count on optimum imaging and you'll want bass traps, maybe diffusing/absorbing panels on the wall to minimize comb filtering. You'll be a lot happier & a lot less effort bringing them out 3 ft or more
Will your dealer let you take do an in-house evaluation with a floor model? Even if it's the older 1.6's, that will give you an excellent opportunity to see if they are the right speaker for you. there is not that much difference between the 1.6 & 1.7 that you wouldn't get a real good idea on how they'd sound.